Tetrafluoroethylene polymers in molded form are useful where low coefficients of friction and good high temperature stability are desirable, such as, for example, bearings. These molding powders sometimes contain a filler such as glass fibers, bronze particles or powder, and the like, to improve their mechanical properties such as wear resistance.
These molding powders are ordinarily fabricated by preforming in a mold under pressure at room temperature, followed by sintering the preform at a temperature above the melting point of the polymer, e.g., at about 370.degree.-400.degree. C. The sintering can be carried out either while the preform is in the mold, or after the preform has been removed from the mold.
In the case of bronze-filled molding powder, there is a problem with discoloration of the molding to an unattractive purple-black color in those parts of the molding which are sintered under air-starved conditions. Even when sintering is carried out in air, there can be air-starved regions at the center of large moldings where air does not penetrate. When the molding is cut in machining a part there is a very unattractive difference in color in different regions of the part. It is believed that the discoloration is caused by degradation of the tetrafluoroethylene polymer at high temperature to give small amounts of carbon, the degradation being catalyzed by the bronze.
In the case of glass-filled molding powder, there is a problem also. When the filler is glass in any of its forms such as fiber, beads, etc., in regions air-starved during sintering, there is a general gray discoloration and localized black or dark gray spots. It is believed that the color and the specks are caused by iron in the glass. It is believed that this impurity promotes the decomposition of some of the polymer to elemental carbon and that the dark spots or specks are deposits of elemental carbon.
In any event, if the preform is removed from the mold prior to sintering, so that its surface is exposed to the atmosphere during sintering, no discoloration or dark specks appear on the surface. However, if the preform is thick enough that the air cannot penetrate into the interior of the preform then during sintering the air-starved interior will become gray and contain dark specks and upon subsequent cutting, the exposed interior reveals the unsightly specks.
Similarly, if the preform is not removed from the mold prior to sintering, but rather remains in the mold during sintering, then air cannot reach the surface of the preform and upon removal of the sintered preform from the mold, the discoloration and specks are present on the surface of the molded articles as well as in the interior.
Removal of impurities from glass fiber fillers can be accomplished by cleaning the glass before preparing the filled tetrafluoroethylene resin. However, this introduces a separate step into the process, and furthermore, only removes the impurities from one of several possible sources of contamination. A means for preventing the dark spots and discoloration without any additional process steps is desirable.